There are many types and appearances of disc herniation's (protrusions, extrusions and sequestered disc fragments), and disc herniation's occur in athletes and the general population in about the same frequency according to the Northeast Collaborative Group on Low Back Pain.
One thing that all disc herniation's have in common, is that they all occur as a result of an annular tear. The annulus is like a retaining wall and an annular tear is a weak spot or defect in that portion of the “the wall” that is supposed to retain the central nuclear material.
The tear is a defect or channel that allows the nuclear material to escape from the center portion of the disc and move out to the periphery (protrusion). If there is enough pressure generated inside the disc, the nuclear material may be squeezed sufficiently to even escape the confines of the disc partially (extrusion) or completely to become free floating (sequestered). This process of annular tearing and then movement of disc material through the defect to the periphery of the disc certainly explains why back pain can be the first thing that people feel before they feel pain goes down the legs (sciatica). The back pain is felt because of the annular injury. As the nuclear material moves toward the periphery of the disc or outside the disc, then pain may be felt in a part if the thigh, leg or foot that corresponds with the nerve being touched by the disc fragment. Think of these nerves like electrical wires passing from the back to the legs and feet. If the wires are contacted by something (disc fragment), then the wire signals trouble in the form of pain, numbness, or weakness. The term “radicular pain” refers to the pain that radiates or is felt in a part of the buttock, thigh or leg because of this nerve stimulation, not because there is something wrong with with the leg.
The disc injury can lead to two obvious sources of pain. One, local back pain from the annular injury and two, pain that radiates (radicular) to the leg or foot from pressure exerted on a nerve root.
Think of the disc like the center of an Oreo cookie that can pooch out on any side depending on where the pressure is applied. In the cookie example there are 360 degrees around the cookie that the disc can protrude. In the lumbar spine, we are concerned primarily with the back half of the cookie or the back (posterior) 180 degrees, since there aren’t really any nerves to push on in the front half or 180 degrees. If the disc herniates in the front half of the disc, there are no nerves to push on.
The way the nerves travel in the spinal canal allow them to come in contact with the disc at various places and come into contact with various discs. So we have various discs that can press against various nerves in various places that give rise to different patterns of pain.
This whole business is further confused by the fact that not all protruding discs are going to cause symptoms, and some discs will press on more than one nerve at a time So which disc protrusion is causing which problem? This where experience counts along with an intimate understanding of the anatomy. The classical problems are not hard to figure out, it’s the problems that don’t quite fit the picture that are challenging, especially when one is trying to limit the amount of surgery that they might be exposed to.
An example might help. The most common levels for disc herniation are L4-5 and L5-S1 which account for around 90% of symptomatic herniation's. At the L5-S1 level, the only nerves that can be affected are L5 and the sacral nerves (primarily S1). However at the L4-5 level, any nerve that passes by this level (L4, L5, and S1) can be affected, although usually it is L5 nerve root.
The nerves can encounter a disc herniation in the central canal or in the foraminal canal. The nerves are said to “traverse” in the central canal or “exit” through the foraminal canal. The nerves can be thought of as electrical wires or small garden hoses with water passing through. If a disc is herniated and pushes on the hose, the location where the flow of water is blocked will determine what is hurting. So in the example where there is a large herniated disc at L4-5, it is possible that even S1 could be affected as it passes by.
A very severe situation of nerve compression in the lumbar spine is referred to as cauda equina syndrome where all of the nerves passing by a large herniated disc are compressed to the point where there is loss of bowel and/or bladder control along with loss of sensation and “radicular” complaints in the lower extremities.